So its time to get my JCM900 2500 SLX back up and running

Rebuilding a JCM900 2500 SLX: stock or mod?

  • Keep it stock, Marshall got it right

    Votes: 5 55.6%

  • Mod a few things to correct some factory mistakes

    Votes: 4 44.4%

  • Gonzo this thing and change it entirely

    Votes: 0 0.0%

  • What the hell do I care?

    Votes: 0 0.0%
  • Total voters
    9
Clockworkmike

Clockworkmike

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This is the longest I've gone with having something completely down and out of service, so its well past it's time to have my poor dead amp back up on its feet. The minimum requirements are to:
1) Have it handled by a real professional: not by my hands since I feel I have only done more damage than good ( lots of things im great at doing, this ain't one of them and I have no shame in admitting that lol). I applaud all those that CAN do this.

2) To have it completely checked down from top to bottom with a fine tooth comb and address any and all present or even future issues that can be seen, regardless of costs. Its my first tube amp and I've had it since I was 13. Its worth saving in my opinion, if for no other reason than nostalgia.

3) Here comes the poll topic: is it better to leave this completely stock as it was made back in 1993 or is this the time to potentially make changes or "mods". If yes to the mods: what should be done? What could be done to take it the next level? Im not necessarily hunting to turning this into some ungodly gain monster and a one trick pony by virtue. But, if there is something (s) to give this an edge and make it go over the top, I'm open to the idea.

I know by merely asking this, I'm opening up the possibility of conflicting arguments on here from the tech bunch but its worth asking lol
 
Far Rider said:
I've always believed that a Marshall should stay a Marshall. Modding them seems like an insult to their designers.
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I largely agree with this philosophy: an amp was built the way it was intended to be.

I guess then a better question would be: if an amp came out with manufacturing defects like something was initially built wrong from the factory (i.e wrong resistor values, undersized components etc) is that technically modding or fixing a well known mistake?
 
I've got a pair of the Marshall JCM 900 Hi Gain Master Volume MKIII's. The 50 watt and 100 watt that preceeded the SLX. I liked the gain, but not the particular sound of the diode clipping when you brought the gain up. I think if Marshall had chosen differently they might have come out much better. Needless to say, I ripped all the diodes out and completely redesigned the preamp with a lot of reading AMS' old posts over at MF. I really like the way they turned out. However, you don't have diodes. I would fix what is broken, recap it and probably leave it alone. Oh, I would definitely roll tubes in it. I would look at some vintage preamp tubes like GE, RCA, Sylvania and the like. Maybe try a 5751 in a position or two.
 
Cadorman said:
I've got a pair of the Marshall JCM 900 Hi Gain Master Volume MKIII's. The 50 watt and 100 watt that preceeded the SLX. I liked the gain, but not the particular sound of the diode clipping when you brought the gain up. I think if Marshall had chosen differently they might have come out much better. Needless to say, I ripped all the diodes out and completely redesigned the preamp with a lot of reading AMS' old posts over at MF. I really like the way they turned out. However, you don't have diodes. I would fix what is broken, recap it and probably leave it alone. Oh, I would definitely roll tubes in it. I would look at some vintage preamp tubes like GE, RCA, Sylvania and the like. Maybe try a 5751 in a position or two.
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I did have a 12AU7 in it for a while in V2. Killed some of the initial gain but increased the overall headroom. I had considered trying a 5751 to cut some of that extremism down
 
I expect you never intend to sell this amp since it was your first, but consider this: I bought the pair of my 900s for a grand including shipping a few years ago. If I would have just stuck to recapping, cleaning, replacing a few bad pots and knobs I could probably sell them now and double or triple my money. As it is, I like what I have done, but I definitely would not feel right about selling them to someone without major disclosure in person and probably for pennies on the dollar.
 
Cadorman said:
I expect you never intend to sell this amp since it was your first, but consider this: I bought the pair of my 900s for a grand including shipping a few years ago. If I would have just stuck to recapping, cleaning, replacing a few bad pots and knobs I could probably sell them now and double or triple my money. As it is, I like what I have done, but I definitely would not feel right about selling them to someone without major disclosure in person and probably for pennies on the dollar.
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Very good point. Modding often depreciates value unfortunately. Rarely, does it increase it unless it was done by some famous guy like Jose, Friedman, Bogner etc and even then its 50/50 if youd get your cost back.

Like you said, doubtful i ever get rid of it, so its irreverent i guess worrying about values and depreciation. But i guess im old school and like to keep things the same lol
 
syscokid said:
"Mod a few things to correct some factory mistakes"
What are the known factory mistakes?
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Not that i have any direct knowledge but ive been told there are some incorrect resistor values throughout this thing. I have seen a few in mine that weren't the same as what the schematics show. Also things like the bridge rectifiers are somewhat undersized and mounted directly to the board, so they get hot and burn. This has already happened on mine with BR3 and a large capacitor (C8) that is directly behind it. It swelled up and began cutting the sound in and out
 
Clockworkmike said:
I largely agree with this philosophy: an amp was built the way it was intended to be.

I guess then a better question would be: if an amp came out with manufacturing defects like something was initially built wrong from the factory (i.e wrong resistor values, undersized components etc) is that technically modding or fixing a well known mistake?
Click to expand...
There are no wrong resistor values.
But I admit the audio coupling caps could have been 600 volt.
People have misinterpreted the circuit and tried to modify it based on purely imaginative conclusions.

The best thing to do is put fresh caps on the circuit board, and play it.

Geniuses on the internet, that tell you to modify it : you should pretty much ignore them.
 
Clockworkmike said:
2) To have it completely checked down from top to bottom with a fine tooth comb and address any and all present or even future issues that can be seen, regardless of costs. Its my first tube amp and I've had it since I was 13. Its worth saving in my opinion, if for no other reason than nostalgia.
Click to expand...
I definitely like this idea. If you got somebody that you can trust, then this is the way to go. Beefing up power resistors that really need beefing up and elevating the ones that run hot (If they're slammed against the board) is a no brainer. That goes the same for any cap that's not rated for twice the voltage of what the circuit is hitting it with. Pots, switches, and all soldering points need to be carefully scrutinized too.
 
The resistors are fuses to limit the current.
In case the intended current is exceeded, the resistor is supposed to blow.
If the tube draws too much current the resistor is supposed to blow.
This prevents damage to the rest of the amp and the output transformer.
Increasing rating of resistor prevents the exceeded current limit from blowing the resistor.

There is one thing you can do, because the hot resistor may be laying on top of the fiberglass.
You can use elevated resistors to keep the heat away from the board.
But if the wattage rating is increased too much, the protection is disabled.
If a 4 watt resistor is called for, than replace it with a 4 watt resistor.

1626651259222.png Like this, it keeps the heat away from the fiberglass board.

The purpose of resistors is to limit current.
If a resistor in a circuit burns up, it means something in the circuit was drawing too much current.
and the resistor blew...as intended.

But it does not mean that the resistor wattage size should be increased.

You should be finding the fault that causes over current....and burning the resistor.
But you should not be assuming that the resistor is under sized.

The screen grid resistors in any tube power amp are: current limiting resistors.
They are supposed to blow if the tube is shorted, protecting the transformers.
But put in a larger wattage resistor --- and the protection is now bypassed.

The people who designed these amps were not idiots.
 
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Amp Mad Scientist said:
The people who designed these amps were not idiots
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Of course not. But some designer's designs are influenced by non-designers... aka: Beancounters! Input jacks soldered directly to a circuit board is an example. In the example of tube sockets that are soldered directly to the circuit board, I seen some that are supported properly and others where the circuit board is flexing a lot everytime a tube is removed or inserted. IMO, 1/2 watt rated resistors used for dropping the B+ rail is not a good idea either.
 
By the suggestions, I think it's settled then and decided: the amp stays 100% stock and no changes, except if its to safely increase its longevity and beef up any weak areas

I'm totally fine with this personally and to be perfectly honest: I'm looking forward to having it back as originally designed. By this, I mean, is that I bought this "New" from a very crooked guitar shop back in 1996 when i was just 13 ( saved up my pesos from working with my dad aka child labor lol) but unfortunately, it was years later that I discovered that it in fact was a used 1993 model once I ran the serial numbers from the chassis, the board and ultimately confirmed it with Marshall themselves.

This is why i suspect values in components aren't the same from what i see and thus, someone's hands have been all thru this poor thing. Its hard to say if I've even been playing the amp Im SUPPOSED to have at this point really.

But i got a guy, Mr. charlie Powers from a repair shop in Fairmont, Wv called Guitar CPR ( runs an equally interesting youtube channel of the same name). He's the only one in this entire area I even remotely trust in fixing this and doing it right. I'd gladly hand it over to a number of you all right here at TTR, but im literally on the other side of the country lol
 
Amp Mad Scientist said:
There are no wrong resistor values.
But I admit the audio coupling caps could have been 600 volt.
People have misinterpreted the circuit and tried to modify it based on purely imaginative conclusions.

The best thing to do is put fresh caps on the circuit board, and play it.

Geniuses on the internet, that tell you to modify it : you should pretty much ignore them.
Click to expand...

What if a blithering idiot, such as myself, suggests modding it???
 
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